Most refrigeration systems are not 100% free of leaks. Thus, the amount (or mass) of refrigerant within the refrigeration system decreases over time. Refrigeration systems, however, are designed to operate with a specific amount of refrigerant. Therefore, loss of refrigerant in a refrigeration system over time typically reduces the efficiency of the refrigeration system. Also, if the amount of refrigerant in the refrigeration system drops to a  certain level, the refrigeration system may cease to operate and/or be damaged. For these reasons, a common maintenance operation for a refrigeration system is to recharge the refrigerant within the refrigeration system.
The recharging operation typically involves flushing the refrigeration system of any remaining refrigerant and, if present, other materials within the refrigeration system. Once the refrigeration system is flushed, a predetermined amount of new refrigerant is introduced into the refrigeration system. An important component of conventional refrigeration charging systems is the device that measures the amount of refrigerant introduced into the refrigeration system. Since the refrigeration system is designed to operate with a specific amount of refrigerant, too little or too much refrigerant can reduce the effectiveness of the recharging operation.
One conventional device used to measure the amount of refrigerant introduced into the refrigeration system is a load cell (or scale). Essentially, the load cell measures the weight of a tank containing the refrigerant before the refrigerant is introduced into the refrigeration system and then afterwards. The difference between the two readings is the amount of refrigerant introduced into the refrigeration system. There are, however, certain problems that are associated with the use of a load cell in conventional recharging systems. Load cells are sensitive to vibration, which can throw off the measurements. Also, since the load cell determines the weight of the entire tank used to supply the refrigerant, which can weigh several hundred pounds or more, sensitivity of the load cell is reduced.
The recharging of refrigeration systems has been complicated by the changes in refrigeration systems and refrigerants used in the refrigeration systems. Recently, the refrigerant in automobile air conditioning systems has been switched from chlorofluorocarbon 12 (CFC-12) to hydrofluorocarbon 134a (HFC-134a) for the purpose of ozone layer protection. However, HFC-134a is considered to be a contributor to global warming, and new types of refrigerant are being considered, of which carbon dioxide (CO2) is one. Compared to HFC-134a, carbon dioxide only has about 1/1300 the global warming potential. However, the use of carbon dioxide presents issues not seen with conventional refrigeration systems. For example, carbon dioxide has an operation pressure that is 7 to 10 times higher than that of HFC-134a. The resulting increase in pressure necessitates thicker walls for the equipment handling the carbon dioxide, such as the storage tank for the refrigeration recharging system. These thicker walls add additional weight to the tank, which further exacerbates the previously discussed problems, such as the sensitivity of the load cell.
Another conventional device used to measure the amount of refrigerant introduced into the refrigeration system employs mass flow technology. As recognized by those skilled in the art, mass flow technology implements a sensor that measures the flow rate of fluid (i.e., the refrigerant) flowing past a certain point. However, use of mass flow technology is very expensive, and the expense is even greater when an increased sensitivity for calculating the amount of refrigerant delivered is desired. The accuracy of mass flow technology is dependent on the fluid state since the mass flow sensor does not measure gas as well as liquid. Although most refrigerants are in a liquid form during the recharging operation, carbon dioxide is in a gaseous state during a recharging operation. Also, mass flow technology does not work well with two-phase fluids. Thus, when carbon dioxide, or another gaseous refrigerant, is used, the sensitivity of the mass flow technology is reduced. There is, therefore, a need for a refrigerant charging system and method that is more accurate and vibration-resistant, and less expensive than conventional refrigeration charging systems, particularly when the refrigerant is a gaseous refrigerant such as carbon dioxide.